chown: detect Solaris and FreeBSD bug

chown: detect Solaris and FreeBSD bug

Solaris 9 and FreeBSD 7.2 chown("link-to-file/",uid,gid)
mistakenly changes ownership of "file".

* lib/chown.c (rpl_chown): Work around bug.
* m4/chown.m4 (gl_FUNC_CHOWN): Check for trailing slash bugs.
(gl_PREREQ_CHOWN): Delete.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add witness.
* modules/unistd (Makefile.am): Populate it.
* lib/unistd.in.h (chown): Update declaration.
* lib/lchown.c (chown): Update client.
* modules/lchown (Depends-on): Add lstat.
* doc/posix-functions/chown.texi (chown): Document the bug.
* doc/posix-functions/getgroups.texi (getgroups): Document
getgroups pitfall.
* modules/chown-tests: New file.
* tests/test-chown.h (test_chown): Likewise.
* tests/test-chown.c (main): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>